This invention relates to a glass composition having a low sodium permeability. More particularly, this invention relates to a fused quartz or synthetic silica composition doped with aluminum, yttrium, cesium or mixtures thereof. The glass composition of the invention is particularly well suited to be used in the formation of an arc chamber in sodium containing metal halide lamps.
Throughout the specification, numerous references will be made to the use of the glass composition to form the arc Chamber of a sodium containing metal halide lamp. However, it should be realized that the inventive glass composition is also suited for use in other lamp applications and the semiconductor industry.
Metal halide arc discharge lamps in which the glass composition of this invention is beneficial when utilized to form the arc chamber include, but are not limited to U.S. Pat. Nos. 4,047,067 and 4,918,352 (electrode), and 5,032,762 (electrodeless), the disclosures of which are herein incorporated by reference. Metal halide lamps of this type are generally comprised of an arc discharge chamber surrounded by a protective envelope. The arc chamber includes a fill of light emitting metals including sodium and rare earth elements such as scandium, indium, dysprosium, neodymium, praseodymium, cerium, and thorium in the form of a halide, optionally mercury, and optionally an inert gas such as krypton or argon. U.S. Pat. No. 4,798,895, herein incorporated by reference, describes a representative metal halide dose which when used in combination with an envelope comprised of sodium resistant glass of the present invention, creates a superior lamp.
Particularly, it has been found that the life of metal halide lamps is frequently limited by the loss of the sodium portion of the metal halide fill during lamp operation via sodium ion diffusion through the arc chamber. More particularly, fused quartz and synthetic silica are relatively porous to a sodium ion, and during lamp operation, energetic sodium ions pass from the arc plasma through the chamber wall and condense in the region between the arc chamber and the outer jacket or envelope of the lamp. The lost sodium is then unavailable to the arc discharge and can no longer contribute its characteristic emissions, causing the light output to gradually diminish, and causing the color to shift from white towards blue. In addition, the arc becomes more constricted, and in a horizontally operated lamp, the arc may bow against and soften the arc chamber wall. Sodium loss may also cause the operating voltage of the lamp to increase to the point where the arc can no longer be sustained by the ballast and failure of the lamp may result.
In an attempt to reduce the effects of sodium diffusion through the arc chamber, the skilled artisan has historically relied on coating the arc chamber with sodium diffusion resistant materials. Attempts to solve diffusion problems have included depositing aluminum silicate and titanium silicate layers on the outside surfaces of the arc tube, as described in U.S. Pat. Nos. 4,047,067 and 4,017,163 respectively. Alternatively, U.S. Reissue Pat. No. 30,165 discloses applying a vitreous metal phosphate and arsenate coating on the inner surface of the arc tube. In contrast, U.S. Pat. No. 5,032,762 discloses beryllium oxide coatings.
While these methods have met with success in reducing sodium diffusion, the methods also require additional processing steps associated with applying a coating. Furthermore, the lamp's high temperature of operation, and frequently corrosive environment, may destroy the adherence between coating and arc chamber substrate. Moreover, cracking and/or peeling can result, exposing the quartz to sodium ions and allowing sodium diffusion to occur. Accordingly, it would be desirable in the art to have a glass material which reduces sodium diffusion without the application of additional coatings.